1. Field of the Invention
The present invention relates to corrugating adhesives which contain a composition comprising tapioca fiber in combination with corn fiber and/or spent germ flake as a replacement for starch in the carrier. More particularly, the invention relates to high speed corrugating adhesives which are prepared by incorporating a composition comprising tapioca fiber in combination with corn fiber and/or spent germ flake in place of normally used corn starch in carrier type adhesive formulations to obtain a product which provides the benefits normally associated with using modified starch in the carrier. The invention provides excellent finished paste viscosity stability and permits a high level of solids in the finished paste.
2. Description of the Related Art
U.S. Pat. No. 5,358,559 issued to Fitt et al. provides a detailed description of the components and manufacturing process involved in producing corrugated products. Corrugated products, such as corrugated board which can be formed into various shapes, such as boxes, are made of corrugated medium bonded to flat sheets of paper. "single-facer" board comprises one sheet of flat paper bonded to the corrugated medium and "double-backer" or "double-facer" is produced when one flat sheet is bonded to the other side of the corrugated medium.
The Fitt et al. patent describes the adhesive used in the corrugating process as generally comprising starch, caustic, a boron containing compound and, where water resistance is needed, a water-proofing agent. As the main binder of the adhesive composition of corrugating paper, the starch is gelatinized in the corrugating process as it penetrates the paper fiber. The remaining ingredients modify the basic properties of the starch. Starch-based adhesives can be of the carrier, no-carrier and carrier-no-carrier type. The adhesive is normally applied to one or both sides of the corrugated medium and paper is attached thereto and is bonded by pressure and heat. The viscosity of the adhesive allows for the paper to be set in place prior to the heat and pressure bonding. The speed at which a corrugating machine can operate is in part limited by the strength of the early adhesion, called the green bond, of the paper to the corrugating medium.
The patent discloses that fibers can be added to the starch-based corrugating adhesives to enhance adhesion and dispersion and yield improved adhesive characteristics including increased waterproofness, dry strength, viscosity and adhesiveness. The patent also discloses that hemicellulose, a natural and readily available component of corn kernels and hulls can be used to enhance the adhesiveness of starch-based corrugating adhesives. U.S. Pat. No. 4,941,922 is distinguished in the specification of the Fitt et al. patent as employing relatively large fibers such as natural fibers including wood, paper, cotton and rayon, synthetic fibers including Nylon, polyester, polypropylene, and Lycra Spandex and metallic fibers which are said to physically intertwine and thereby enhance adhesive properties.
U.S. Pat. Nos. 4,826,719, 4,677,145, 4,600,739, and 4,094,718 disclose starch based corrugating adhesives which further comprise polyvinyl alcohol to increase the viscosity and strength of the bond between the corrugating medium and the liner material. Noting certain limitations in these disclosures, U.S. Pat. No. 5,075,360 to Fitt et al. disclosed a carrier-type corrugating adhesive wherein the carrier phase is prepared by further hydrolyzing a cold water soluble, partially hydrolyzed, polyvinyl alcohol in situ in the presence of water, caustic and starch, modified starch or dextrin.
The entire disclosures of these patents are hereby incorporated into the present disclosure by reference thereto.
It has now been discovered in connection with the present invention that a combination of tapioca fiber and corn fiber or tapioca fiber and spent germ flake can be used in place of corn starch in corrugating adhesives. The tapioca fiber, which generally can contain from about 35% to about 70% by weight of tapioca starch, has been found to thin, losing viscosity, in high alkaline conditions. This is in contrast to corn fiber which thickens under such conditions. The use of tapioca fiber and corn fiber or spent germ flake in place of corn starch permits the corrugator to use less material in formulating the adhesive and still achieve similar or stronger adhesive properties. The use of unmodified or pearl starch in carrier type corrugating adhesive formulations is limited to no more than 200 pounds in conventional equipment because it tends to build too much viscosity in the primary portion of the mix when more is used. The relatively small amount of gelled primary solids limits the total solids in the adhesive paste to a maximum of about 23-24% dry basis (db). By contrast, when modified starch is used in the carrier type adhesives, less viscosity is built at the same solids level as pearl starch and, therefore, amounts of 250-400 pounds of modified starch can be used in the carrier portion of the mix. This provides for increases in the total solids in the finished paste of approximately 30-33%, which accounts for most of the functional benefits of modified starch carriers. The use of the combination of tapioca fiber and corn fiber or tapioca fiber and spent germ flake in place of pearl or modified starch allows for the use of amounts of material similar to the starch formulations yet provides a paste having the functional benefits of modified starch formulations.